Method of producing semiconductor epitaxial wafer, semiconductor epitaxial wafer, and method of producing solid-state image sensor

ABSTRACT

A production method for a semiconductor epitaxial wafer includes: a first step of irradiating a surface of a semiconductor wafer with cluster ions to form a modified layer that is located in a surface portion of the semiconductor wafer and that includes a constituent element of the cluster ions in solid solution; and a second step of forming an epitaxial layer on the modified layer of the semiconductor wafer. The first step is performed such that a portion of the modified layer in terms of a thickness direction becomes an amorphous layer and an average depth of an amorphous layer surface at a semiconductor wafer surface-side of the amorphous layer is at least 20 nm from the surface of the semiconductor wafer.

TECHNICAL FIELD

The present disclosure relates to a method for producing a semiconductorepitaxial wafer, a semiconductor epitaxial wafer, and a method forproducing a solid-state image sensor.

BACKGROUND

Metal contamination is one of the factors that cause deterioration insemiconductor device characteristics. For example, in the case of aback-illumination solid-state image sensor, metal that is mixed into asemiconductor epitaxial wafer used as a substrate of the image sensor isa factor causing increased dark current in the solid-state image sensorand causing defects referred to as white spot defects. In aback-illumination solid-state image sensor, a wiring layer and the likeare provided in a lower layer than a sensor section such that externallight can be directly taken in by the sensor, enabling clear images andvideos to be recorded even in dark locations. For this reason,back-illumination solid-state image sensors have become widely used inrecent years in digital video cameras and mobile telephones such assmart phones. Therefore, it is desirable to reduce white spot defects toas great an extent as possible.

Mixing of a metal into a wafer mainly occurs during a process ofproducing a semiconductor epitaxial wafer and a process of producing asolid-state image sensor (device production process). Metalcontamination in the former process of producing a semiconductorepitaxial wafer may for example occur due to heavy metal particles fromconstituent materials of an epitaxial growth furnace or heavy metalparticles produced through metal corrosion of piping materials as aresult of a chlorine-containing gas being used in the furnace duringepitaxial growth. The metal contamination described above has beenimproved to a certain extent in recent years through replacement of theconstituent materials of epitaxial growth furnaces with materials havingsuperior corrosion resistance; however, this improvement is stillinsufficient. On the other hand, in the latter process of producing asolid-state image sensor, heavy metal contamination of a semiconductorsubstrate is a concern during various processing steps such as ionimplantation, diffusion, and oxidizing heat treatment.

One technique for inhibiting heavy metal contamination such as describedabove involves providing a gettering site in a semiconductor wafer forcapturing heavy metals. In a known method using this technique, ions areimplanted in a semiconductor wafer and an epitaxial layer issubsequently formed. In this method, a region into which the ions areimplanted functions as a gettering site.

PTL 1 describes a semiconductor epitaxial wafer production methodincluding: a first step of irradiating a surface of a semiconductorwafer with cluster ions to form a modified layer that is located in asurface portion of the semiconductor wafer and that includes aconstituent element of the cluster is ions in solid solution; and asecond step of forming an epitaxial layer on the modified layer of thesemiconductor wafer.

CITATION LIST Patent Literature

PTL 1: WO 2012/157162

SUMMARY Technical Problem

PTL 1 demonstrates that higher gettering ability can be obtained througha modified layer formed by irradiation with cluster ions than through anion implantation region formed by implantation of monomer ions (singleions). However, the present inventor recognized a new technical problemas described below. Specifically, the gettering ability of the modifiedlayer in PTL 1 can for example be effectively increased by increasingthe dose of cluster ions. However, the inventor ascertained thatexcessively increasing the dose causes a large number of epitaxialdefects to occur in the subsequently formed epitaxial layer. PTL 1 doesnot consider how to achieve a balance of both improving getteringability and suppressing occurrence of epitaxial defects, which leavesroom for improvement in addressing this issue.

In light of the problem described above, an objective of the presentdisclosure is to provide a semiconductor epitaxial wafer that has highgettering ability and in which occurrence of epitaxial defects issuppressed, and also to provide a production method for thissemiconductor epitaxial wafer.

Solution to Problem

The inventor reached the following findings as a result of furtherinvestigation.

(1) When a semiconductor wafer is irradiated with cluster ions, anamorphous region may or may not be formed in the modified layerdepending on the irradiation conditions. Moreover, higher getteringability can be obtained when an amorphous layer is present in a portionof the modified layer in terms of a thickness direction than when anamorphous region is not present in the modified layer. In other words,in order to obtain high gettering ability, it is necessary to carry outcluster ion irradiation under conditions that lead to formation of anamorphous layer in a portion of the modified layer in terms of thethickness direction.

(2) Epitaxial defects may occur due to damage near the surface (surfaceportion) of the semiconductor wafer resulting from cluster ionirradiation. The inventor discovered that the dose at which epitaxialdefects start to occur differs depending on the type of cluster ionsthat are used. In other words, the inventor realized that occurrence ofepitaxial defects is not dependent solely on the dose.

(3) As a result of further investigation, the inventor discovered thatthere is a correlation between the depth position of the amorphous layerin the modified layer and occurrence of epitaxial defects. Specifically,it is necessary to form the amorphous layer at at least a prescribeddistance from the semiconductor wafer surface in order to suppressoccurrence of epitaxial defects.

(4) Thus, in order to achieve a balance of high gettering performanceand suppression of epitaxial defects, it is necessary for an amorphouslayer to be formed in the modified layer by cluster ion irradiation andfor the amorphous layer to be formed at at least a certain depth. Theinventor also discovered that when an epitaxial layer is formed aftercluster ion irradiation, crystallinity of the modified layer recoversdue to heat during this epitaxial layer formation, which leads to lossof the amorphous layer and occurrence of black spot defects in themodified layer. Furthermore, the inventor realized that in asemiconductor epitaxial wafer in which epitaxial defects occur due tothe amorphous layer being at a shallow depth position, the number ofblack spot defects described above increases excessively such thatrather than being “black spot defects”, these defects actually form aline-shaped defect layer of connected black spots. In other words, inthe case of a semiconductor epitaxial wafer in which black spot defectsare present in the modified layer, high gettering ability can beobtained and occurrence of epitaxial defects can be suppressed.

The key points of the present disclosure based on the above findings areas follows.

A method of producing a semiconductor epitaxial wafer according to thepresent disclosure includes:

a first step of irradiating a surface of a semiconductor wafer withcluster ions to form a modified layer that is located in a surfaceportion of the semiconductor wafer and that includes a constituentelement of the cluster ions in solid solution; and

a second step of forming an epitaxial layer on the modified layer of thesemiconductor wafer,

wherein the first step is performed such that a portion of the modifiedlayer in terms of a thickness direction becomes an amorphous layer andan average depth of an amorphous layer surface at a semiconductor wafersurface-side of the amorphous layer is at least 20 nm from the surfaceof the semiconductor wafer.

The first step is preferably performed such that the average depth is atleast 20 nm and no greater than 200 nm from the surface of thesemiconductor wafer.

Moreover, the first step is preferably performed such that an averagethickness of the amorphous layer is no greater than 100 nm.

The cluster ions preferably include carbon as a constituent element andmore preferably include at least two elements as constituent elements ofwhich one is carbon. The cluster ions preferably have a carbon number ofno greater than 16.

A semiconductor epitaxial wafer according to the present disclosureincludes: a semiconductor wafer; a modified layer located in a surfaceportion of the semiconductor wafer and including a prescribed element insolid solution within the semiconductor wafer; and an epitaxial layerlocated on the modified layer, wherein black spot defects are present inthe modified layer.

The black spot defects are preferably present at a depth of at least 30nm from a surface of the semiconductor wafer. The black spot defectspreferably have a width of from 30 nm to 100 nm and a density of from1.0×10⁸ defects/cm² to 1.0×10¹⁰ defects/cm².

The prescribed element preferably includes carbon and more preferablyincludes at least two elements of which one is carbon.

A method of producing a solid-state image sensor according to thepresent disclosure includes forming a solid-state image sensor in theepitaxial layer of the semiconductor epitaxial wafer produced by any oneof the producing methods described above or of any one of thesemiconductor epitaxial wafers described above.

Advantageous Effect

The semiconductor epitaxial wafer production method according to thepresent disclosure can provide a semiconductor epitaxial wafer that hashigh gettering ability and in which occurrence of epitaxial defects issuppressed. The semiconductor epitaxial wafer according to the presentdisclosure has high gettering ability and occurrence of epitaxialdefects therein is suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A, 1B, and 1C are cross-sectional views schematicallyillustrating a method of producing a semiconductor epitaxial wafer 100according to an embodiment of the present disclosure;

FIG. 2A is a schematic view illustrating an irradiation mechanism inirradiation with cluster ions and FIG. 2B is a schematic viewillustrating an implantation mechanism in implantation of monomer ions;

FIG. 3 is a graph illustrating a relationship between dose and averagedepth of an amorphous layer surface and a relationship between dose andepitaxial defect density when acceleration voltage and beam current arefixed in a situation in which C₃H₅ cluster ions are used;

FIG. 4 is a graph illustrating a relationship between dose and averagedepth of an amorphous layer surface and a relationship between dose andepitaxial defect density when acceleration voltage and beam current arefixed in a situation in which C₃H₃ cluster ions are used;

FIG. 5 shows TEM images of cross-sections of epitaxial silicon wafers(i.e., after epitaxial layer growth) in an experiment shown in FIG. 3,wherein FIG. 5A shows a situation in which the dose was 1.0×10¹⁵atoms/cm² (comparative example), FIG. 5B shows a situation in which thedose was 2.0×10¹⁵ atoms/cm² (example), and FIG. 5C shows a situation inwhich the dose was 3.0×10¹⁵ atoms/cm² (comparative example); and

FIG. 6 shows TEM images of cross-sections of modified layers aftercluster ion irradiation and before epitaxial layer formation in theexperiment shown in FIG. 3, wherein FIG. 6A shows a situation in whichthe dose was 1.0×10¹⁵ atoms/cm² (comparative example), FIG. 6B shows asituation in which the dose was 1.7×10¹⁵ atoms/cm² (example), FIG. 6Cshows a situation in which the dose was 2.0×10¹⁵ atoms/cm² (example),and FIG. 6D shows a situation in which the dose was 3.0×10¹⁵ atoms/cm²(comparative example).

DETAILED DESCRIPTION

The following provides a detailed description of embodiments of thepresent disclosure with reference to the drawings. Note that thicknessesof a modified layer 14, an amorphous layer 16, and an epitaxial layer 18are exaggerated relative to a semiconductor wafer 10 in FIG. 1 in orderto facilitate explanation and thus the ratio of thicknesses in FIG. 1differs from the actual ratio.

Semiconductor Epitaxial Wafer Production Method

As illustrated in FIG. 1, a production method for a semiconductorepitaxial wafer 100 according to an embodiment of the present disclosureincludes: a first step (FIGS. 1A and 1B) of irradiating a surface 10A ofa semiconductor wafer 10 with cluster ions 12 to form a modified layer14 that is located in a surface portion of the semiconductor wafer 10and that includes a constituent element of the cluster ions 12 in solidsolution; and a second step (FIG. 1C) of forming an epitaxial layer 18on the modified layer 14 of the semiconductor wafer 10. FIG. 1C is across-sectional view schematically illustrating the semiconductorepitaxial wafer 100 obtained as a result of this production method. Theepitaxial layer 18 forms a device layer for production of asemiconductor element such as a back-illumination solid-state imagesensor.

The semiconductor wafer 10 is for example a bulk monocrystalline waferthat is made from silicon or a compound semiconductor (GaAs, GaN, orSiC) and that does not have an epitaxial layer at the surface. In thecase of production of a back-illumination solid-state image sensor, thesemiconductor wafer 10 is normally a bulk monocrystalline silicon wafer.The semiconductor wafer 10 can for example be obtained by growing amonocrystalline silicon ingot through the Czochralski method (CZ method)or the floating zone melting method (FZ method) and slicing the ingotusing a wire saw or the like. Carbon and/or nitrogen may be added to thesemiconductor wafer 10 in order to obtain higher gettering ability.Furthermore, a freely selected dopant may be added to the semiconductorwafer 10 in a prescribed concentration to obtain an n+ type, p+ type, n−type, or p− type substrate.

Alternatively, the semiconductor wafer 10 may be an epitaxialsemiconductor wafer in which a semiconductor epitaxial layer has beenformed on the surface of a bulk semiconductor wafer. For example, thesemiconductor wafer 10 may be an epitaxial silicon wafer in which asilicon epitaxial layer has been formed on the surface of a bulkmonocrystalline silicon wafer. The silicon epitaxial layer can be formedthrough CVD under normally used conditions. The thickness of theepitaxial layer is preferably in a range from 0.1 μm to 10 μm and morepreferably in a range from 0.2 μm to 5 μm.

Herein, a cluster ion irradiation step illustrated in FIG. 1A is acharacteristic step of the present embodiment. The technicalsignificance of adopting this step is explained in addition to theaction and effect thereof. The modified layer 14 formed as a result ofirradiation with the cluster ions 12 is a region in a surface portion ofthe semiconductor wafer at which constituent elements of the clusterions 12 are localized in solid solution at interstitial positions orsubstitution positions of the crystal. The modified layer 14 functionsas a gettering site. The reason for this is thought to be as follows.Specifically, an element such as carbon that is irradiated in the formof cluster ions becomes localized in high density at interstitialpositions or substitution positions in the silicon monocrystal.Furthermore, it has been confirmed experimentally that when carbon orthe like in solid solution within the silicon monocrystal reaches anequilibrium concentration or greater, the solid solubility of heavymetals (saturation solubility of transition metals) increasessignificantly. In other words, dissolving carbon or the like in solidsolution at the equilibrium concentration or greater is thought toincrease the solid solubility of heavy metals such that the capture rateof these heavy metals is dramatically increased.

Note that the present description uses the term “cluster ion” to referto an ionized product formed by applying positive charge or negativecharge to a cluster of a plurality of atoms or molecules that areassembled into a single mass. The cluster is a lump-shaped group ofatoms or molecules (normally from approximately 2 to 2,000) that arebound to one another.

Irradiation with the cluster ions 12 in the present embodiment allowshigher gettering ability to be obtained than in a situation in whichmonomer ions are implanted. The inventor considers the action by whichthis effect is achieved to be as follows.

In a situation in which, for example, carbon monomer ions are implantedin a silicon wafer, the monomer ions knock against silicon atoms formingthe silicon wafer and become implanted at a prescribed depth position inthe silicon wafer as illustrated in FIG. 2B. The implantation depth isdependent on the type of constituent element of the implanted ions andthe acceleration voltage of the ions. In the situation described above,the depth direction concentration profile of carbon in the silicon waferis relatively broad and a region in which carbon is implanted extendsfor roughly 0.5 μm to 1 μm. When different types of ions are irradiatedsimultaneously with the same energy, lighter elements tend to beimplanted deeper. In other words, elements are implanted at differentpositions in accordance with their mass such that the implanted elementshave a broader concentration profile.

On the other hand, in a situation in which a silicon wafer is irradiatedwith cluster ions formed, for example, from carbon and hydrogen, uponirradiation with the cluster ions 12 as illustrated in FIG. 2A, theenergy of the cluster ions 12 causes the silicon wafer to momentarilyreach a high temperature state of approximately 1,350° C. to 1,400° C.and melts the silicon. Thereafter, the silicon cools rapidly such thatthe carbon and hydrogen dissolve in solid solution near the surface ofthe silicon wafer. In other words, the “modified layer” in the presentdescription is a layer in which constituent elements of the irradiatedions are in solid solution at interstitial positions or substitutionpositions in the crystal within a surface portion of the semiconductorwafer. The depth direction concentration profile of carbon in thesilicon wafer is dependent on the acceleration voltage and the clustersize of the cluster ions. However, the concentration profile is sharperthan in the case of monomer ions and the thickness of a region in whichthe irradiated carbon is localized (i.e., the modified layer) is roughlyno greater than 500 nm (for example, approximately 50 nm to 400 nm).Note that an element that is irradiated in the form of cluster ionsundergoes a certain degree of thermal diffusion during formation of theepitaxial layer 18. Consequently, the concentration profile of carbonafter epitaxial layer 18 formation has a broad diffusion region on eachside of a peak indicating where the aforementioned elements arelocalized. However, the thickness of the modified layer (i.e., the widthof the peak) does not change significantly. Therefore, a localized andhigh concentration carbon deposition region can be formed. Moreover,formation of the modified layer 14 near the surface of the siliconwafer, and thus directly under the epitaxial layer 18, enables proximitygettering. It is thought that high gettering ability can be obtained asa result. Note that different types of ions may be irradiatedsimultaneously so long as the ions are in the form of cluster ions.

Furthermore, a feature of the present embodiment is that irradiationwith cluster ions is performed such that, as illustrated in FIG. 1B, anamorphous layer 16 is formed in a portion of the modified layer 14 interms of the depth direction and an average depth of an amorphous layersurface 16A at a semiconductor wafer surface-side of the amorphous layer16 is at least 20 nm from the semiconductor wafer surface 10A. Thegettering ability of the modified layer 14 described above can beimproved as a result of the amorphous layer 16 being present in themodified layer 14. Therefore, a back-illumination solid-state imagesensor produced from the semiconductor epitaxial wafer 100 that isobtained according to the present embodiment is expected to benefit fromsuppression of white spot defects. An average depth of at least 20 nmfrom the semiconductor wafer surface 10A for the surface 16A of theamorphous layer 16 allows sufficient suppression of occurrence ofepitaxial defects in the subsequently formed epitaxial layer 18.

From a viewpoint of more effectively suppressing epitaxial defects, theaverage depth of the surface 16A of the amorphous layer 16 is preferablyat least 20 nm and no greater than 200 nm from the semiconductor wafersurface 10A, and more preferably at least 20 nm and no greater than 80nm from the semiconductor wafer surface 10A.

The average thickness of the amorphous layer 16 is preferably no greaterthan 100 nm and more preferably no greater than 60 nm. An averagethickness of greater than 100 nm may make it difficult to select clusterirradiation conditions that ensure the depth of the surface 16A is atleast 20 nm from the semiconductor wafer surface 10A.

Note that as illustrated in FIG. 1B and also FIGS. 6A-6D explainedfurther below, the depth of the amorphous layer surface varies dependingon the width direction position. In the present disclosure, the “averagedepth of the amorphous layer surface at the semiconductor wafersurface-side of the amorphous layer” is defined as the average depth ofthe surface in a transmission electron microscopy (TEM) image obtainedthrough observation of a cross-section of the amorphous layer using aTEM. The “average depth” is an intermediate depth between a shallowestposition and a deepest position of a boundary between the amorphouslayer and a crystalline region. The “average thickness of the amorphouslayer” is defined as the average thickness of the amorphous layer in aTEM image and, more specifically, the difference between the averagedepths of two surfaces of the amorphous layer. The magnification of theTEM image should be of a level that enables the amorphous layer to beclearly observed; in the examples illustrated in FIG. 6, themagnification is ×500,000.

The cluster ions may include a variety of clusters depending on thebinding mode and can be generated by commonly known methods such asdescribed, for example, in the following documents. Gas cluster beamgeneration methods are described in (1) JP H9-41138 A and (2) JPH4-354865 A. Ion beam generation methods are described in (1) ChargedParticle Beam Engineering, Junzo Ishikawa, ISBN 978-4-339-00734-3,Corona Publishing Co., Ltd.; (2) Electron/Ion Beam Engineering, TheInstitute of Electrical Engineers of Japan, ISBN 4-88686-217-9, Ohmsha.Ltd.; and (3) Cluster Ion Beam Fundamentals and Applications, ISBN4-526-05765-7, Nikkan Kogyo Shimbun, Ltd. In general, a Nielsen ionsource or a Kaufman ion source is used for generating positively chargedcluster ions, whereas a high current negative ion source using volumegeneration is used for generating negatively charged cluster ions.

The following describes conditions of the cluster ion irradiation.

No specific limitations are placed on elements used for irradiationother than being elements that contribute to gettering. Examples ofelements that can be used include carbon, boron, phosphorous, andarsenic. However, it is preferable that the cluster ions include carbonas a constituent element from a viewpoint of obtaining higher getteringability. Carbon atoms at lattice positions have a small covalent radiuscompared to the silicon monocrystal and form contraction sites in thesilicon crystal lattice, which results in high gettering ability ofattracting interstitial impurities.

The irradiated elements are preferably at least two elements of whichone is carbon. In particular, it is preferable that one or more dopantelements selected from the group consisting of boron, phosphorus,arsenic, and antimony are irradiated in addition to carbon. The types ofmetals that can be effectively gettered differ depending of the types ofelements that are in solid solution. Therefore, providing at least twoelements in solid solution makes it possible to deal with a wider rangeof metal contaminants. For example, nickel can be effectively getteredusing carbon, whereas copper and iron can be effectively gettered usingboron.

No specific limitations are placed on compounds that are ionized.Examples of carbon source compounds that can be ionized include ethane,methane, and carbon dioxide (CO₂), whereas examples of boron sourcecompounds that can be ionized include diborane and decaborane (B₁₀H₁₄).For example, in a situation in which a mixed gas of dibenzil anddecaborane is used as a material gas, hydrogenated compound clusters inwhich carbon, boron, and hydrogen are aggregated can be produced.Alternatively, in a situation which cyclohexane (C₆H₁₂) is used as amaterial gas, cluster ions formed from carbon and hydrogen can beproduced. Clusters C_(n)H_(m) (3≦n≦16, 3≦m≦10) produced from pyrene(C₁₆H₁₀), dibenzil (C₁₄H₁₄), or the like are particularly preferable asa carbon source compound. The reason for the above is that a beam ofsmall cluster ions can be easily controlled.

The compound that is ionized is preferably a compound that includes bothcarbon and the previously mentioned dopant element. When a compound suchas described above is irradiated as cluster ions, carbon and the dopantelement can both be provided in solid solution through a singleirradiation.

Whether or not an amorphous layer is formed in the modified layer andthe average depth of the surface 16A of the amorphous layer 16 whenformation does occur are controlled through cluster ion dose, clustersize, cluster ion acceleration voltage, beam current, and so forth, andare particularly dependent on the dose and the cluster size. The presentdescription uses the term “cluster size” to refer to the number of atomsor molecules forming a single cluster.

The cluster size can be set as appropriate as from 2 to 100atoms/molecules, preferably no greater than 60 atoms/molecules, and morepreferably no greater than 50 atoms/molecules. In the examples describedfurther below, C₃H₅ having a cluster size of 8 and C₃H₃ having a clustersize of 6 were used. The cluster size can for example be adjusted byadjusting the pressure of gas sprayed from a nozzle, the pressure in avacuum vessel, and the voltage applied to a filament during ionization.Note that the cluster size can be calculated by obtaining a clusternumber distribution by mass spectrometry using a quadrupole highfrequency electric field or time-of-flight mass spectrometry and takingan average value of the cluster numbers.

The dose of cluster ions can be adjusted by controlling the ionirradiation time. In the present embodiment, the dose is required to beroughly at least 1×10¹⁵ atoms/cm² in order to form the amorphous layer16 in the modified layer 14. In the examples described further below, anamorphous layer was formed in a modified layer through a carbon dose ofat least 1.7×10¹⁵ atoms/cm² when C₃H₅ cluster ions were used (refer toFIG. 3) and a carbon dose of at least 2.2×10¹⁵ atoms/cm² when C₃H₃cluster ions were used (refer to FIG. 4). Furthermore, the dose isrequired to be roughly no greater than 1×10¹⁶ atoms/cm ² in order thatthe average depth of the surface 16A at the semiconductor wafersurface-side of the amorphous layer 16 is least 20 nm from thesemiconductor wafer surface 10A. In the examples described furtherbelow, the average depth of the surface 16A was at least 20 nm from thesemiconductor wafer surface 10A through a carbon dose of no greater than2.0×10¹⁵ atoms/cm² when C₃H₅ cluster ions were used (refer to FIG. 3)and a carbon dose of no greater than 2.6×10¹⁵ atoms/cm² when C₃H₃cluster ions were used (refer to FIG. 4).

The acceleration voltage of the cluster ions, in combination with thecluster size, influences the position of a peak in the depth directionconcentration profile of a constituent element in the modified layer 18and thus also indirectly influences the depth of the amorphous layer. Ina situation in which C_(n)H_(m)(3≦n≦16, 3≦m≦10) cluster ions are used,the acceleration voltage per one carbon atom is required to be greaterthan 0 keV/atom and no greater than 50 keV/atom, and preferably nogreater than 40 keV/atom, in order that the average depth of the surface16A at the semiconductor wafer surface-side of the amorphous layer 16 isat least 20 nm from the semiconductor wafer surface 10A.

Adjustment of the acceleration voltage is usually performed by twomethods: (1) electrostatic acceleration and (2) high-frequencyacceleration. The former method is for example a method in whichelectrodes are arranged at equal intervals and the same voltage isapplied between the electrodes such that a constant accelerating fieldis formed in an axial direction. The latter method is for example alinac (linear accelerator) method in which ions are caused to travelalong a straight line while being accelerated by high-frequency waves.

The beam current is required to be roughly at least 100 μA and nogreater than 1,000 μA in order that the amorphous layer 16 is formed inthe modified layer 14 and in order that the average depth of the surface16A of the amorphous layer 16 is at least 20 nm from the semiconductorwafer surface 10A.

The following describes heat treatment in the present embodiment Monomerions are normally implanted with an acceleration voltage ofapproximately 150 keV to 2,000 keV and as a result of each of the ionscolliding with a silicon atom with this energy, crystallinity of asurface portion of the silicon wafer in which the monomer ions areimplanted is disturbed and crystallinity of an epitaxial layersubsequently grown on the wafer surface is also disturbed. On the otherhand, cluster ions are normally irradiated with an acceleration voltageof approximately 10 keV/cluster to 100 keV/cluster and the cluster ionscan be caused to impact with a small energy per one atom or moleculebecause each cluster is an aggregate of a plurality of atoms ormolecules. Consequently, the extent of damage to the semiconductor wafercrystal is small. Therefore, in one embodiment, the semiconductorepitaxial wafer 100 having high gettering ability can be effectivelyproduced by performing the first step and then transferring thesemiconductor wafer to an epitaxial growth apparatus to carry out thesecond step without subjecting the semiconductor wafer to heat treatmentafter the first step to allow recovery of crystallinity. In other words,it is not necessary to perform recovery heat treatment using a rapidheating and cooling heat treatment apparatus such as a rapid thermalannealing (RTA) apparatus or a rapid thermal oxidation (RTO) apparatusthat is separate to the epitaxial apparatus.

The reason for this is that crystallinity of the semiconductor wafer 10can sufficiently recover during hydrogen baking treatment that isperformed in advance of epitaxial growth in the epitaxial apparatus forforming the epitaxial layer 18, which is described further below. Normalconditions for the hydrogen baking treatment involve providing ahydrogen environment in the epitaxial growth apparatus, loading thesemiconductor wafer 10 into a furnace of the apparatus at an internalfurnace temperature of at least 600° C. and no greater than 900° C.,heating at a rate of at least 1° C./s and no greater than 15° C./s to atemperature of at least 1,100° C. and no greater than 1,200° C., andholding the semiconductor wafer 10 for at least 30 seconds and nogreater than 1 minute at the aforementioned temperature. Although theoriginal purpose of this hydrogen baking treatment is to remove anatural oxide film formed on the wafer surface due to washing treatmentprior to epitaxial layer growth, hydrogen baking performed under theconditions described above also enables sufficient recovery ofcrystallinity of the semiconductor wafer 10.

It should be noted that alternatively, recovery heat treatment may ofcourse be performed after the first step and prior to the second stepusing a heat treatment apparatus that is separate from the epitaxialapparatus. The recovery heat treatment is performed for at least 10seconds and no greater than 1 hour at at least 900° C. and no greaterthan 1,200° C. The recovery heat treatment can for example be performedusing a rapid heating and cooling heat treatment apparatus, such as anRTA apparatus or an RTO apparatus, or a batch heat treatment apparatus(vertical heat treatment apparatus or horizontal heat treatmentapparatus) before transferring the semiconductor wafer 10 into theepitaxial growth apparatus.

The epitaxial layer 18 formed on the modified layer 14 is for example asilicon epitaxial layer and can be formed under normally usedconditions.

For example, a source gas such as dichlorosilane or trichlorosilane maybe introduced into a chamber using hydrogen as a carrier gas andepitaxial growth may be performed on the semiconductor wafer 10 by CVDat a temperature in range of roughly 1,000° C. to 1,200° C., althoughthe growth temperature does differ depending on the source gas. Theepitaxial layer 18 preferably has a thickness in a range of from 1 μm to15 μm. A thickness of less than 1 μm may cause a change in resistivityof the epitaxial layer 18 due to outward diffusion of dopant from thesemiconductor wafer 10 and a thickness of greater than 15 μm may affectspectral sensitivity characteristics of the solid-state image sensor.

Semiconductor Epitaxial Wafer

The following describes the semiconductor epitaxial wafer 100 that isobtained through the production method described above. As illustratedin FIG. 1C, the semiconductor epitaxial wafer 100 includes thesemiconductor wafer 10, the modified layer 14 that is located in thesurface portion of the semiconductor wafer 10 and that includes theprescribed element in solid solution within the semiconductor wafer 10,and the epitaxial layer 18 located on the modified layer 14.

The modified layer 14 is defined as previously explained and can beidentified as a steep peak section in a depth direction concentrationprofile of the prescribed element which is obtained by performingelemental analysis in the depth direction from the surface 10A of thesemiconductor wafer 10 using a secondary ion mass spectrometer (SIMS).The modified layer 14 normally extends from the surface 10A of thesemiconductor wafer 10 to a depth in a range of from 50 nm to 400 nmfrom the surface 10A.

As illustrated in FIG. 1C and also FIG. 5B described further below,black spot defects 20 are present in the modified layer 18. The presentdescription uses the term “black spot defect” to refer to a defect thatis observed as a black spot in the modified layer 14 when a cleavedcross-section of the semiconductor epitaxial wafer 100 is observed by aTEM in bright field mode. According to investigation conducted by theinventor, black spot defects only occur in the modified layer 14 afterformation of the epitaxial layer 18 in a situation in which theamorphous layer 16 has been formed in the modified layer 14 afterirradiation with the cluster ions 12. On the other hand, black spotdefects do not occur in the modified layer after formation of theepitaxial layer in a situation in which an amorphous layer has not beenformed in the modified layer.

The mechanism by which black spot defects occur is thought to be asdescribed below. Specifically, it is presumed that in arecrystallization step in which the amorphous layer formed in themodified layer prior to epitaxial layer formation undergoescrystallinity recovery due to heat energy received during epitaxialgrowth, not only silicon atoms, but also cluster elements (for example,carbon atoms) introduced through cluster irradiation, oxygen atoms inthe silicon wafer, and so forth are taken into the recrystallized regionsuch that the recrystallized region is in a composite clustered defectform, which is observed as black spot-shaped defects.

According to investigation by the inventor, high gettering ability isobtained by the semiconductor epitaxial wafer 100 in which the blackspot defects 20 are present. Furthermore, the inventor discovered thatoccurrence of epitaxial defects can be suppressed when the average depthof the amorphous layer surface 16A is at least 20 nm from thesemiconductor wafer surface 10A, whereas a line-shaped defect layer ofconnected black spots is formed when the amorphous layer becomes sothick that the average depth is less than 20 nm, and this line-shapeddefect layer acts as a starting point for formation of epitaxialdefects.

In the present embodiment, the density of epitaxial defects in theepitaxial layer 18 can be kept to no greater than 0.04 defects/cm² as aresult of the average depth of the amorphous layer surface 16A in themodified layer 14 being at least 20 nm from the semiconductor wafersurface 10A prior to formation of the epitaxial layer 18.

From a viewpoint of suppressing occurrence of epitaxial defects, theblack spot defects are preferably present at a depth of at least 30 nmfrom the semiconductor wafer surface.

In terms of size, the black spot defects have a width (wafer radialdirection) of approximately 30 nm to 100 nm and a height (waferthickness direction) of approximately 20 nm to 60 nm. The density of theblack spot defects is preferably from 1.0×10⁸ defects/cm² to 1.0×10¹⁰defects/cm². A density of at least 1.0×10⁸ defects/cm² enables theeffect of suppressing occurrence of epitaxial defects to be sufficientlyrealized. A density of no greater than 1.0×10¹⁰ defects/cm² ensures thata line-shaped defect layer such as described above is not formed.

No specific limitations are placed on the prescribed element other thanbeing an element that is not a main material of the semiconductor wafer(i.e., silicon in the case of a silicon wafer). As explained above, theprescribed element is preferably carbon or at least two elements ofwhich one is carbon.

The semiconductor epitaxial wafer 100 according to the presentembodiment has high gettering ability and occurrence of epitaxialdefects therein is suppressed.

Solid-State Image Sensor Production Method

A solid-state image sensor production method according to an embodimentof the present disclosure includes forming a solid-state image sensor inthe epitaxial layer at the surface of the semiconductor epitaxial waferdescribed above or of a semiconductor epitaxial wafer produced by theproduction method described above (in other words, in the epitaxiallayer 18 at the surface of the semiconductor epitaxial wafer 100). Thesolid-state image sensor obtained through the above-described productionmethod can more sufficiently suppress occurrence of white spot defectsthan conventionally.

EXAMPLES Experimental Example 1 Comparative Example

An n type silicon wafer (diameter: 300 mm, thickness 725 μm, dopant:phosphorus, dopant concentration: 5.0×10¹⁴ atoms/cm³) was prepared froma CZ monocrystalline silicon ingot. Next, a cluster ion generatingapparatus (model: CLARIS, produced by Nissin Ion Equipment Co., Ltd.)was used to generate C₃H₅ clusters from cyclohexane and irradiate thesurface of the silicon wafer with a carbon dose of 1.0×10¹⁵ atoms/cm² toform a modified layer. The acceleration voltage per one carbon atom was23.4 keV/atom and the beam current was 400 μA.

FIG. 6A is an image observed by a TEM of a cross-section around themodified layer after cluster ion irradiation. It can be seen from FIG.6A that an amorphous layer was not formed in the modified layer; notethat an amorphous layer is indicated by white sections such as in FIGS.6B-6D.

Next, the silicon wafer was transferred into a single wafer epitaxialgrowth apparatus (produced by Applied Materials, Inc.) and was subjectedto hydrogen baking treatment in the apparatus for 30 seconds at 1,120°C. Thereafter, a silicon epitaxial layer (thickness: 8 tμm, dopant:phosphorous, dopant concentration: 1.0×10¹⁵ atoms/cm³) was grown on themodified layer of the silicon wafer by CND at 1,150° C. using hydrogenas a carrier gas and trichlorosilane as a source gas. A siliconepitaxial wafer was obtained as a result.

Concentration profiles of carbon and hydrogen were measured by SIMS. Themodified layer could be identified by confirming the presence of a steeppeak within a range of 80 nm from the silicon wafer surface. Across-section around the modified layer of the silicon epitaxial waferwas observed using a TEM. The modified layer is indicated by the blackbelt-shaped section in the image shown in FIG. 5A, but black spotdefects were not observed.

Example

An experiment was conducted in the same way as in the comparativeexample with the exception that the carbon dose was changed to 2.0×10¹⁵atoms/cm². After cluster ion irradiation, a cross-section around amodified layer was observed using a TEM. As shown in FIG. 6C, anamorphous layer was formed in the modified layer. The white section inFIG. 6C indicates the amorphous layer.

Concentration profiles of carbon and hydrogen were measured by SIMSafter epitaxial layer formation. The modified layer could be identifiedby confirming the presence of a steep peak within a range of 80 nm fromthe silicon wafer surface. A cross-section around the modified layer ofthe silicon epitaxial wafer was observed. The modified layer isindicated by the black belt-shaped section in the image shown in FIG. 5Band black spot defects were observed in the modified layer.

Evaluation of Gettering Ability

The surface of the silicon epitaxial wafer produced in each of thecomparative example and the example was deliberately contaminated by aspin coating contamination method using a Ni contaminant liquid and anFe contaminant liquid (each 1.2×10¹³/cm²) and was then subjected to heattreatment for 30 minutes at 900° C. Thereafter, the concentrations of Niand Fe captured in the modified layer were measured by SIMS. The resultsare shown in Table 1.

TABLE 1 Comparative example Example Amount of captured Ni 5.0 × 10¹² 1.1× 10¹³ [atoms/cm²] Amount of captured Fe 1.0 × 10¹² 9.0 × 10¹²[atoms/cm²]

The example exhibited higher gettering ability than the comparativeexample. In the example, it is thought that formation of the amorphouslayer in the modified layer led to recrystallization in a region of theamorphous layer after epitaxial layer formation and that this regionalso contributed as a gettering site.

Experimental Example 2

As illustrated by the plot in FIG. 3, silicon epitaxial wafers wereprepared in the same way as Experimental Example 1 but with differentcarbon doses ranging from 1.0×10¹⁵ atoms/cm² to 1.0×10¹⁶ atoms/cm².

After cluster ion irradiation, a cross-section around each modifiedlayer was observed using a TEM. Measurements were performed to determinewhether or not an amorphous layer was formed in each of the modifiedlayers and, in a situation in which an amorphous layer was formed, todetermine the average depth of the amorphous layer surface at thesemiconductor wafer surface-side of the amorphous layer and the averagethickness of the amorphous layer. Representative examples of TEM imagesare shown in FIG. 6A for when the dose was 1.0×10¹⁵ atoms/cm². FIG. 6Bfor when the dose was 1.7×10¹⁵ atoms/cm², FIG. 6C for when the dose was2.0×10¹⁵ atoms/cm², and FIG, 6D for when the dose was 3.0×10¹⁵atoms/cm². The average depth of the surface at the semiconductor wafersurface-side of the amorphous layer was 55 nm in FIG. 6B, 20 nm in FIG.6C, and 5 nm in FIG. 6D. The average thickness of the amorphous layerwas 5 nm in FIG. 6B, 30 nm in FIG. 6C, and 60 nm in FIG. 6D. FIG. 3illustrates the relationship between the dose and the average depth. Anamorphous layer was not formed. when the dose was less than 1.7×10¹⁵atoms/cm². The average depth was at least 20 nm when the dose was atleast 1.7×10¹⁵ atoms/cm² and no greater than 2.0×10¹⁵ atoms/cm².

The surface of the silicon epitaxial layer of each of the epitaxialsilicon wafers was measured by a Surfscan SP1 (produced by KLA-TencorCorporation) in normal mode and among defects counted as LPDs of atleast 90 nm, each defect counted as an LPD-N was defined as an epitaxialdefect. FIG. 3 illustrates the relationship between the dose and thedensity of epitaxial defects. Epitaxial defects occurred with a densityexceeding 0.04 defects/cm² when the dose exceeded 2.0×10¹⁵ atoms/cm².

The results explained above demonstrate that when an accelerationvoltage per one carbon atom of 23.4 keV/atom and a beam current of 400μA are fixed in a situation in which C₃H₅ cluster ions are used, a doseof at least 1.7×10¹⁵ atoms/cm² enables formation of an amorphous layerand provision of high gettering ability, whereas a dose of no greaterthan 2.0×10¹⁵ atoms/cm² enables an average depth of at least 20 nm andsuppression of epitaxial defects.

Moreover, when the dose was at least 1.7×10¹⁵ atoms/cm² and no greaterthan 2.0×10¹⁵ atoms/cm², black spot defects were observed afterepitaxial layer formation as representatively illustrated in FIG. 5Bdescribed above. Black spot defects were not observed when the dose wasless than 1.7×10¹⁵ atoms/cm², When the dose was greater than 2.0×10¹⁵atoms/cm², black spot defects were not observed, but a line-shapeddefect layer of connected black spots was observed as representativelyillustrated in FIG. 5C.

Table 2 shows the depth of black spot defects from the silicon wafersurface, the width of black spot defects, and the density of black spotdefects for four sets of experimental conditions in which black spotdefects were observed.

TABLE 2 Carbon dose [atoms/cm²] Black spot defects 1.70E+15 1.80E+151.90E+15 2.00E+15 Depth [nm] 58 46 39 30 Width [nm] 31 47 67 79 Density[defects/cm²] 3.8 × 10⁸ 7.4 × 10⁸ 2.4 × 10⁹ 8.9 × 10⁹

Experimental Example 3

An experiment was conducted in the same way as in Experimental Example 2with the exception that the type of cluster ions was changed to C₃H₃clusters generated from cyclohexane. FIG. 4 illustrates the results ofthe experiment. In Experimental Example 3, a dose of at least 2.2×1¹⁵atoms/cm² enabled amorphous layer formation and provision of highgettering ability, whereas a dose of no greater than 2.6×10¹⁵ atoms/cm²enabled an average depth of at least 20 nm and suppression of epitaxialdefects.

Moreover, when the dose was at least 2.2×10¹⁵ atoms/cm² and no greaterthan 2.6×10¹⁵ atoms/cm², black spot defects were observed afterepitaxial layer formation. Black spot defects were not observed when thedose was less than 2.2×10′ atoms/cm². When the dose was greater than2.6×10¹⁵ atoms/cm², black spot defects were not observed, but aline-shaped defect layer of connected black spots was observed.

Table 3 shows the depth of black spot defects from the silicon wafersurface, the width of black spot defects, and the density of black spotdefects for five sets of experimental conditions in which black spotdefects were observed.

TABLE 3 Black spot Carbon dose [atoms/cm²] defects 2.20E+15 2.30E+152.40E+15 2.50E+15 2.60E+15 Depth [nm] 65 54 42 38 32 Width [nm] 30 45 5873 89 Density 1.3 × 10⁸ 6.7 × 10⁸ 1.6 × 10⁹ 4.3 × 10⁹ 9.4 × 10⁹[defects/cm²]

INDUSTRIAL APPLICABILITY

The present disclosure can provide a semiconductor epitaxial waferhaving high gettering ability and in which occurrence of defects in anepitaxial layer is suppressed and can provide a method for producingthis semiconductor epitaxial wafer.

REFERENCE SIGNS LIST

-   100 semiconductor epitaxial wafer-   10 semiconductor wafer-   10A semiconductor wafer surface-   12 cluster ion-   14 modified layer-   16 amorphous layer-   16A surface at semiconductor wafer surface-side of amorphous layer-   18 epitaxial layer-   20 black spot defect

1. A method of producing a semiconductor epitaxial wafer, comprising: afirst step of irradiating a surface of a semiconductor wafer withcluster ions to form a modified layer that is located in a surfaceportion of the semiconductor wafer and that includes a constituentelement of the cluster ions in solid solution; and a second step offorming an epitaxial layer on the modified layer of the semiconductorwafer, wherein the first step is performed such that a portion of themodified layer in terms of a thickness direction becomes an amorphouslayer and an average depth of an amorphous layer surface at asemiconductor wafer surface-side of the amorphous layer is at least 20nm from the surface of the semiconductor wafer.
 2. The method ofproducing a semiconductor epitaxial wafer of claim 1, wherein the firststep is performed such that the average depth is at least 20 nm and nogreater than 200 nm from the surface of the semiconductor wafer.
 3. Themethod of producing a semiconductor epitaxial wafer of claim 1, whereinthe first step is performed such that an average thickness of theamorphous layer is no greater than 100 nm.
 4. The method of producing asemiconductor epitaxial wafer of claim 1, wherein the cluster ionsinclude carbon as a constituent element.
 5. The method of producing asemiconductor epitaxial wafer of claim 4, wherein the cluster ionsinclude at least two elements as constituent elements of which one iscarbon.
 6. The method of producing a semiconductor epitaxial wafer ofclaim 4, wherein the cluster ions have a carbon number of no greaterthan
 16. 7. A semiconductor epitaxial wafer comprising: a semiconductorwafer; a modified layer located in a surface portion of thesemiconductor wafer and including a prescribed element in solid solutionwithin the semiconductor wafer; and an epitaxial layer located on themodified layer, wherein black spot defects are present in the modifiedlayer.
 8. The semiconductor epitaxial wafer of claim 7, wherein theblack spot defects are present at a depth of at least 30 nm from asurface of the semiconductor wafer.
 9. The semiconductor epitaxial waferof claim 7, wherein the black spot defects have a width of from 30 nm to100 nm and a density of from 1.0×10⁸ defects/cm² to 1.0×10¹⁰defects/cm².
 10. The semiconductor epitaxial wafer of claim 7, whereinthe prescribed element includes carbon.
 11. The semiconductor epitaxialwafer of claim 10, wherein the prescribed element includes at least twoelements of which one is carbon.
 12. A method of producing a solid-stateimage sensor, comprising forming a solid-state image sensor in theepitaxial layer of the semiconductor epitaxial wafer produced by theproducing method of claim 1 or of the semiconductor epitaxial wafer ofclaim 7.